The menu bar is a set of menus at the top of each Emacs frame. It is available only when MenuBarMode is on, which it is by default.
See also ToolBar and ScrollBar.
You can disable the menu bar. Even when there is no menu bar, you can access the menu as a popup, using ‘C-mouse-3’.
To disable the menu bar:
(menu-bar-mode -1) to your init file.(set-specifier menubar-visible-p nil) to your init file, or use (set-menubar nil). However, this does not work for buffer-local menu bars (and some things might fail when there is no menu bar).before:

After:
Starting with GnuEmacs 21, you can disable the menu bar but still have it available on a popup menu:
(menu-bar-mode -99)’ – disable the menu-bar‘C-mouse-3’ to pop up the entire menu-bar as a popup menu.For a similar popup functionality for the tool bar, see ToolBarPlus.
‘menubar’ to change the colors of the menu bar. Emacs.pane.menubar.background: darkGrey
Emacs.pane.menubar.foreground: black
Emacs*menubar.margin: 0
Emacs*shadowThickness: 1CategoryDisplay CategoryFrames CategoryHelp CategoryGlossary CategoryMenus